The Trump administration moved on Thursday to remove court-imposed time limits on the detention of migrant children, proposing to end 20 years of judicial oversight and allow families to be held indefinitely in secure facilities as their cases wend through the immigration courts.

The proposed new regulation reflects President Trump’s frustration that thousands of families from Central America and elsewhere are continuing to stream across the southwest border, assured in part by guarantees in the current legal process that migrants who arrive with children will not be held for long periods in detention.

If approved, the new regulation would upend a body of rules for detaining migrant children that has been in place since 1997. Multiple administrations have challenged the rules, which stem from a consent decree in a federal class-action lawsuit over the physical and emotional harm done to children held in jail-like settings for extended periods. As recently as July, those efforts have been rejected by the judge overseeing the case.

Rather than try its hand again in court, the Trump administration proposed on Thursday to withdraw entirely from the consent decree, saying that it would replace the agreement with regulations that would honor its spirit, and ensure that all migrant children in government custody are “treated with dignity, respect and special concern for their particular vulnerability as minors.”

The decree was meant to be temporary until it could be codified into law. However, in announcing the proposed new rule, the administration said that it planned to change elements of the agreement that the advocates who negotiated it saw as fundamental — namely, a 20-day limit on detaining the families in immigration jails, after which they must be released unless they opt out by choice.

“Today, legal loopholes significantly hinder the department’s ability to appropriately detain and promptly remove family units that have no legal basis to remain in the country,” said the secretary of homeland security, Kristjen Nielsen. “This rule addresses one of the primary pull factors for illegal immigration and allows the federal government to enforce immigration laws as passed by Congress.”

Peter Schey, president of the Center for Human Rights and Constitutional Law, who was a leader of the legal team in the original case, said he was already preparing to challenge the government in court. He and other advocates called the announcement on Thursday a thinly veiled attempt at an end run around the courts.

“The Trump administration has been whittling away at the basic rights of women and children since they came into office,” said Michelle Brané, director of the migrant rights and justice program at the Women’s Refugee Commission. “Efforts to weaken or eliminate basic child protection standards by calling them a burden or loopholes, and eliminating their obligations for the basic care of children, is just another example of the administration’s abdication of human rights.”

“The court has already had to to step in repeatedly to uphold these basic child welfare principles,” Ms. Brané added. “It is clear that the administration is incapable of holding themselves accountable.”

The Trump administration has argued that the consent decree, known as the “Flores” agreement, has encouraged migrants to travel north by the thousands, and to bring children along to shield themselves from the threat of lengthy detention.

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Unaccompanied migrant children at a shelter in Houston. An administration proposal to bypass restrictions on the detention of migrant families is expected to end up in court.CreditMarian Carrasquero for The New York Times

But immigrant advocates contend that most migrants are bringing their children to escape violence and poverty, not as bargaining chips to avoid detention. “Border apprehension data over the past twenty years shows that the Flores settlement has had nothing to do with the number of refugees seeking to enter the United States from Central America,” Mr. Schey said. He pointed to federal data showing that unauthorized crossings along the border with Mexico have sharply declined over the past two decades.

“Treating children humanely and not detaining them indefinitely in often intolerable conditions is not a legal loophole, as the secretary of homeland security claims,” Mr. Schey said. “It is the way civilized nations treat innocent children.”

Currently, migrants who cross the border without permission, including those seeking asylum, are funneled into a system of detention centers — ranging from county jails to nonsecure shelters that some have compared to summer camp — depending on the migrants’ age and family status.

Adults who cross the border alone can be held indefinitely in jails. Children who cross alone can also be held indefinitely, though only in nonsecure shelters, meaning that the children can technically leave at any time (though they are strongly discouraged from doing so).

The big dilemma facing the administration is what to do about adults who illegally cross the border with children.

Families in such cases are typically placed in federally run detention centers that are outfitted to house children and adults together, but they can only be held there for up to 20 days. At that point, because of the consent decree, the families are often released, sometimes with GPS ankle bracelets, until their cases are settled in immigration court.

The government said it would develop a network of licensed facilities that can humanely shelter migrant families in the months or longer it takes for their deportation or asylum cases to be heard. But it provided scant details on how the facilities would operate, or why the new plan might pass muster with the court when previous attempts to ease limits on the detention of migrant children have not.

The 1997 consent decree was reached after advocates successfully argued that federal detention was damaging, physically and emotionally, to children’s health and limited their access to legal counsel. In 2016, the courts extended the settlement to apply not only to children who were crossing the border alone, but to migrant families.

The issue became newly controversial early this year, when Attorney General Jeff Sessions installed a zero-tolerance border policy, in which the government sought to jail and prosecute every adult who crossed the border without permission. For months, migrant families were separated as adults were placed in detention, and children were sent to less-restrictive shelters.

A federal court ordered that the families be reunited, and the Justice Department also lost its bid for a reprieve from the Flores agreement, a move which would have allowed the zero-tolerance enforcement policy to continue by holding children in detention, together with their parents.

In a ruling that countered nearly every argument the government posed, Judge Dolly M. Gee of the Federal District Court in Los Angeles called the government’s motion “a cynical attempt” to shift immigration policymaking to the courts in the wake of “over 20 years of congressional inaction and ill-considered executive action that have led to the current stalemate.”

The new proposal announced Thursday is also likely to end up back in court. The public has 60 days to comment on the proposed rules, followed by a 45-day period in which lawyers who negotiated the original settlement can challenge the government’s move before Judge Gee.

A version of this article appears in print on , on Page A20 of the New York edition with the headline: U.S. Aims to Ease Rules on Detaining Children. Order Reprints | Today’s Paper | Subscribe